224 DEPTHS OF THE OCEAN chap. 



two different worlds, the Atlantic world south of the ridge, the 



Arctic world north of it. 

 Decrease of The surface-tempcrature is naturally high in the equatorial 



temperature Tegious, decreasing toward the poles, where it falls below o° C. 

 from equator Kriimmel has calculated the mean surface-temperatures for each 

 to poles. lo-degree zone throughout the great ocean basins, his figures 



for the North Adantic being : — 



Zone . . o^-io" io°-20° 2o°-30° 3o''-4o"" 40^-50° 5o''-6o" 6o°-7o° N. lat. 

 Temp. . . 26.83 25.60 23.90 20.30 12.94 8.94 4.26 X. 



It is interesting to compare this horizontal distribution of 

 temperature with the vertical distribution in tropical waters. 

 The following temperatures, for instance, were recorded by the 

 German Antarctic Expedition in July 191 1, at a station in lat. 

 7|-° N. in the middle of the Atlantic : — 



At a depth of 100 metres the temperature is seen to be the 

 same as the average surface-temperature in about 40° N. ; the 

 mean surface-temperature at 50° N. is the same as that found at 

 200 metres in the tropics, and the mean surface-temperature at 

 60° N. corresponds to the temperature at a depth of 700-800 

 metres in the tropics. In other words, we have a horizontal 

 distribution of temperature from the equator towards the poles 

 similar to what we have vertically from the surface towards the 

 bottom in the tropics. Near the equator one need only send a 

 thermometer down to 800 metres in order to find the same 

 temperature that one would have to travel 60° northwards to 

 find at the surface, but the other physical conditions are widely 

 different. In the deep water at the equator there is an 

 enormous pressure and unchanging darkness, but at the surface 

 far north and south there is a pressure of only one atmosphere 

 and good light, at least in summer. Thus the physical condi- 

 tions in the deep layers of the tropical waters are really very 

 different from those at the surface towards the poles, and in 

 consequence the conditions of life also differ ; organisms living 

 in the surface-layers of high latitudes are found in far deeper 

 water in low latitudes, in so much as they are capable of adapting 

 themselves to the excessive pressure and the infinitesimal 

 quantity of light. Some organisms seem to be mainly depen- 

 dent on the degree of light, the temperature being of less 

 importance to them. We shall return to the questions of light 



